1. Field of Invention
The present invention relates to a multi-channel variable optical attenuator suitable for the wavelength division multiplexing (WDM) system.
2. Description of Related Prior Art
In the WDM system, optical communications can be made in a large capacity and at a high speed by transmitting wavelength multiplexed light comprising a plurality of signal lights each having a different wavelength in a 1.55um band through an optical fiber accompanied by an optical amplifier. Even if each of signal lights output from a transmitting station is of the same optical level, their optical levels at the receiving station may be varied due to the gain slope of the optical amplifier or the transmission characteristics of the optical fiber. The variation among the optical levels of the signal lights causes an erroneous transmission. Accordingly, it is desirable that the optical levels of respective signal lights should be equalized.
Otsuka et al. has presented an optical de-mulitplexer and an variable optical attenuator which is individually provided for each of de-multiplexed signal lights in the Electronics Society Conference of IEICE (the Institute of Electronics, Information and Communication Engineers) 1997, B-10-101. In this optical attenuator, the attenuation amount of respective signal lights is controlled based on both of the incoming and the outgoing wavelength multiplexed light.
Nakata et al. has presented a variable optical attenuator comprising two polarizing plates, a Faraday rotator, a permanent magnet and an electromagnet in the same Conference of IEICE 1997 C-3-48. In this attenuator, the attenuation amount depends on the Faraday rotation angle controlled by the magnetic field. To use this attenuator in the WDM system and to equalize the optical levels of the signal lights, the same number of the attenuator as that of the signal lights are necessary.
Kawai et al. has presented in the same Conference 1997 B-10-61 that a variable optical attenuator provided with a Mach-Zender interferometer and a heating device to heat up one of paired waveguides of the Mach-Zender interferometer. In this attenuator, the attenuation is performed by varying the phase of the light transmitting in the heated waveguide. To use this type of attenuator in the WDM system, a plurality of attenuators are necessary, too.
In the conventional examples mentioned above, the same number of monitoring devices as that of the signal lights are needed to equalize the optical levels of respective signal lights because the individual attenuator is controlled by the corresponding monitoring device. Consequently, the volume of the equipment, which contains the multi-channel attenuator, will be large and the additive control circuitry will be complicated. Moreover, it will take a long time to equalize the optical levels of respective signal lights when such levels are varied signal to signal or fluctuate.